The United Nations have warned that the alleged use of chemical weapons in Mosul, if confirmed, would be a war crime and a serious violation of international humanitarian law.

In a statement released on Saturday, the UN said that twelve people, including women and children, are being treated for possible exposure to chemical weapons agents in Mosul, where Islamic State is fighting off an offensive by U.S.-backed Iraqi forces.

Four of them are showing "severe signs associated with exposure to a blister agent."

The alleged attack occurred this week in eastern Mosul, an area that was declared fully liberated by Iraqi forces in January. The attack hit a neighborhood along the Tigris River.

Doctors in an urgent care hospital in the nearby city of Erbil, the capital of Iraq's Kurdish region, say they began receiving patients showing symptoms of chemical weapons exposure on Thursday.

The U.N. Humanitarian Coordinator in Iraq, Lise Grande, called for an investigation, saying that:

"This is horrible. If the alleged use of chemical weapons is confirmed, this is a serious violation of international humanitarian law and a war crime, regardless of who the targets or the victims of the attacks are."

The statement released by the International Committee of the Red Cross (ICRC) did not say which side used the chemical agents that caused blisters, redness in the eyes, irritation, vomiting and coughing.

IS has used chemical weapons in Iraq and Syria at least 52 times according to a report published late last year by IHS conflict monitor, a London-based research and intelligence gathering group. The report said that at least 19 of the 52 attacks took place in and around Mosul.

Iraqi forces captured the eastern side of Mosul in January after 100 days of fighting and launched their attack on the districts that lie west of the Tigris river. The eastern side remains within reach of the militants' rockets and mortar shells, with the majority of western Mosul still under Islamic State group control.